Semiconductor packages having metal composite base plates

ABSTRACT

Semiconductor packages are provided that have a base plate with a matrix of pure silver or a silver alloy and reinforcement particles. The reinforcement particles can include high thermal conductivity, low CTE particles selected from the group consisting of diamond, cubic boron nitride (c-BN), silicon carbide (SiC), and any combinations thereof. In some embodiments, the base plate is entirely comprised of the composite. In other embodiments, the base plate has a core made of the composite. The core can include at least one outer layer on the core. The semiconductor package can include one or more dice or transistors on the base plate, an insulated frame on the base plate, and one or more leads on the insulated frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/666,249, filed Jun. 29, 2012 the contents of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure is related to air cavity packages. Moreparticularly, the present disclosure is related to air cavity packageshaving high thermal conductivity base plates and methods for making suchbase plates.

2. Description of Related Art

Semiconductor dice and transistors are often packaged or enclosed toprotect the die or transistor from one or more environmental conditions.

One such package includes an air cavity package, which typicallyincludes one or more semiconductor dice or transistors mounted directlyon a base plate and an insulated frame surrounding the die ortransistor. A lid is placed over the frame, sealing the die ortransistor in a cavity of air. In addition to the structural aspect ofthe base plate for mounting the dice and transistors, the base plate isused to assist in removing heat from the die or transistor byconduction.

Air cavity packages are widely used to house high frequency devices suchas radio-frequency or RF dice and transistors. The power densities ofsuch dice and transistors continue to increase. Thus, it has beendetermined by the present disclosure that there is a need for increasesin the heat dissipating capacity of packages, particularly of baseplates.

In use, air cavity packages undergo numerous high temperature heatcycles. Thus, in addition to having a high heat dissipating capacity,the base plates must also have a coefficient of thermal expansion (CTE)that matches those of the die or transistor and the frame. MinimizingCTE mismatch among the package components is desired to mitigate failureand fatigue of the components that can result from temperature cycling.

Accordingly, there is a continuing need for air cavity packages and baseplates that overcome, alleviate, and/or mitigate one or more of theaforementioned and other deleterious effects of prior art packages.

SUMMARY

A base plate is provided that has a high thermal conductivity. In someembodiments, the base plate is made of reinforced silver composite. Thecomposite can include a matrix of pure silver or a silver alloy andreinforcement particles. The reinforcement particles can include highthermal conductivity, low CTE particles selected from the groupconsisting of diamond, cubic boron nitride (c-BN), silicon carbide(SiC), and any combinations thereof.

A semiconductor package is provided that includes a base plate that isat least partially comprised of a composite made of silver-diamond or asilver alloy-diamond. In some embodiments, the base plate is entirelycomprised of the composite. In other embodiments, the base plate has acore made of the composite. The core can include at least one outerlayer on the core.

The semiconductor package can include one or more dice or transistors onthe base plate, an insulated frame on the base plate, and one or moreleads on the insulated frame.

The above-described and other features and advantages of the presentdisclosure will be appreciated and understood by those skilled in theart from the following detailed description, drawings, and appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an exemplary embodiment of an air cavitypackage according to the present disclosure;

FIG. 2 is a sectional view of the air cavity package of FIG. 1 takenalong line 2-2;

FIG. 3 is a plan view of an exemplary embodiment of a base plateaccording to the present disclosure for use in the air cavity package ofFIG. 1; and

FIG. 4 is a plan view of an exemplary embodiment of a pair of coresaccording to the present disclosure for use in the air cavity package ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings and in particular to FIGS. 1 through 3, anexemplary embodiment of an air cavity package according to the presentdisclosure is shown and is generally referred to by reference numeral10. Advantageously, package 10 includes a base plate 12 having a highthermal capacity, which is made at least partially of reinforced silvercomposite.

Package 10 is shown prior to assembly of a lid (not shown). Package 10includes base plate 12, one or more dice or transistors 14, and aninsulated frame 16. Frame 16 is secured to base plate 12 by a securingmaterial 18. Securing material 18 can be any desired securing materialsuch as, but not limited to, a cured epoxy, a braze, an adhesive, or anyother securing material. Insulated frame 16 generally surrounds the diceor transistors 14 and has one or more conductive leads 20 thereon forelectrical communication with the dice or transistors 14 by way of oneor more wire leads (not shown).

Dice or transistors 14 can include, but are not limited to, laterallydiffused metal oxide semiconductor (LDMOS) dice, field effecttransistors (FET), and high electron mobility transistors (HEMT), andother active semiconductor devices. The semiconductor device can includedevices made of Si, GaAs, SiC, GaN and alloys based upon GaN.Additionally, dice or transistors 14 can also include passive devicessuch as, but not limited to, resistors, terminators, attenuators, andcapacitors.

Base plate 12 includes a core 22 having a high thermal conductivity,namely a thermal conductivity of at least 400 watts per meter Kelvin(W/m-K) at 20 degrees Celsius (° C.), preferably at least 500 W/m-K at20° C. Core 22 is made of a composite comprised of a matrix of puresilver or a silver alloy and reinforcement particles. The reinforcementparticles include diamond, c-BN, SiC, and any combinations thereof. Core22 acts as a heat spreader and provide improved heat removal and lowthermal expansion, as well as an electrical connection for the die.

In some embodiments, base plate 12 can further include one or more outerlayers 24 surrounding core 22. Layer 24 can be made of any conductivematerial such as, but not limited to, copper, silver, nickel, CuAg, CuAgeutectic, gold, platinum, palladium, silver/tungsten, CuMo composite, orW, Mo, Kovar and any combinations or alloys thereof. Preferably, layer24 is made of gold or silver. In some embodiments, the CuAg eutectic canhave a composition of less than 100% weight percent Ag to more than 50%weight percent Ag and any subranges therebetween. In one exemplaryembodiment, the CuAg eutectic has a composition of about 72% weightpercent Ag and about 28% weight percent Cu.

Thus, base plate 12 may be entirely comprised of the composite or thebase plate may include core 22 made of the composite surrounded by oneor more layers 24.

The composite of the present disclosure is comprised of particles ofdiamond, c-BN, SiC and combinations thereof particles within a silver ora silver alloy matrix. In some embodiments, the diamond particles may beaugmented by additional of high thermal conductivity graphite fibers,carbon nanotubes, graphene, c-BN, SiC and any combinations thereof.

In some embodiments having core 22, the layers 24 are thin layersapplied to the core, where the core itself is configured in the desiredshape of base plate 12 with the layers merely adding a thin electricallyconductive skin to the core.

In other embodiments having core 22, the core is configured in a shapecorresponding to the region of base plate 12 that receives dice ortransistors 14 and the layers 24 form the remaining sections of the baseplate as seen in FIG. 3.

It is further contemplated by the present disclosure for base plate 12to include an outer region 26 as seen in FIG. 4. Here, core 22 is in theregion of base plate 12 that receives dice or transistors 14 and isformed of the composite of the present disclosure, while outer region 26forms the remaining regions of the base plate and can be formed of adifferent material.

For example, outer region 26 can be formed of the composite of thepresent disclosure or can be formed of any other thermally conductivematerial such as, but not limited to, copper, silver, CuAg, CuAgeutectic, gold, platinum, palladium, silver/tungsten, CuMo composite, orW, Mo, Kovar and any combinations or alloys thereof. Although notillustrated in FIG. 4, it is further contemplated by the presentdisclosure for this embodiment to include one or more layers 24 coveringand securing the core 22 and outer region 26.

In one particular embodiment, core 22 is formed of the composite havingthe silver-diamond or silver alloy-diamond, while outer region 26 is asingle continuous phase of the silver or silver alloy used in theformation of core 22 but without the diamond. It is contemplated forthis embodiment to optionally include one or more layers 24.

Without wishing to be bound by any particular theory, it is believed bythe present disclosure that silver, which has a low oxygen content, hasa very high affinity for oxygen in the molten state, which has beenfound to act as a positive factor for binding with diamond.

Thus in one embodiment, the composite of the present disclosure isformed by mixing diamond particles with molten silver, which has a lowoxygen content, to provide the desired silver-diamond composite with avery high adhesion between the silver and the diamond. As used herein,the term low oxygen content silver shall mean silver having less than300 parts per million (ppm) of oxygen, with less than 200 ppm beingpreferred, and less than 100 ppm being most preferred. In one particularembodiment of the present disclosure, silver or silver alloy having lessthan 50 ppm oxygen is used.

In some embodiments, the composite includes a further adhesion promotercoated on the diamond particles and, when present, conductivity graphitefibers, carbon nanotubes, graphene, and any combinations thereof.

For example, it is contemplated by the present disclosure for thediamond or other particles to be coated with an oxide such as but notlimited to, TiO₂. The oxide coated diamond and other particles, whenpresent, are then combined with molten silver, which has a low oxygencontent, to provide the desired silver-diamond composite with a veryhigh adhesion between the silver and the diamond.

It is also contemplated by the present disclosure for the diamond orother particles to be coated with one or more layers of Cr, W, Mo, Co,Cu, Ti, Si, SiC, TiN, TiC, Ta, Zr, and any combinations thereof.

Moreover, it has been found by the present disclosure that silver alloyswell with copper, zinc, tin, indium, bismuth and has limited or nosolubility with refractory metals such as tungsten and molybdenum. Thus,the composite of the present disclosure is not limited to silver-diamondeither in the coated or uncoated form, but can also includesilver-alloy-diamond composites such as, but not limited toCuAg-diamond, AgCuSn-diamond, and InCuAg-diamond, where the diamond canbe in coated or uncoated form. In some embodiments, the silver alloy canbe CuAg eutectic.

The silver-diamond or silver alloy-diamond composite of the presentdisclosure advantageously provides superior reflectance of infraredradiation. Thus, base plate 12 can, in some embodiments, provide areflectivity of 95%-99% in the visible spectrum.

In a method of making the silver-diamond composite, diamond particlesand other particles when present, in either coated or uncoated form aremixed with a dry-processing binder and compacted in a die under pressureto form a compacted body. The compacted body of particles can be heatedto evaporate or decompose at least part of the binder and/or causebonding or partial sintering of the particles.

The compacted body and low oxygen silver or silver alloy are then heatedto melt and draw the silver into the bonded or partially sintereddiamond particles. The composite is then cooled and cut to the desiredshape of base plate 12 or core 22. The hot molten silver can be drawninto the bonded particles using a gravity feed, vacuum enhanced gravityfeed, vacuum casting, pressure casting, isostatic pressure, metalinjection molding, and centrifugal casting.

When base plate 12 further includes outer layer 24, core 22 can becoated with one or more of the layers of desired material. When baseplate 12 further includes additional cores, core 22 and the additionalcores can be coated with one or more layers 24 of desired material.

It should also be noted that the terms “first”, “second”, “third”,“upper”, “lower”, and the like may be used herein to modify variouselements. These modifiers do not imply a spatial, sequential, orhierarchical order to the modified elements unless specifically stated.

While the present disclosure has been described with reference to one ormore exemplary embodiments, it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted for elements thereof without departing from the scope of thepresent disclosure. In addition, many modifications may be made to adapta particular situation or material to the teachings of the disclosurewithout departing from the scope thereof. Therefore, it is intended thatthe present disclosure not be limited to the particular embodiment(s)disclosed as the best mode contemplated, but that the disclosure willinclude all embodiments falling within the scope of the presentdisclosure.

What is claimed is:
 1. A semiconductor package comprising a base plateat least partially comprised of a metal matrix composite comprised ofhigh thermal conductivity, low thermal expansion particles in a matrixof silver, wherein the metal matrix composite is formed by mixing thehigh thermal conductivity, low thermal expansion particles with moltensilver that has an oxygen content of less than 300 parts per million. 2.The semiconductor package of claim 1, wherein the silver comprises asilver alloy.
 3. The semiconductor package of claim 2, wherein thesilver alloy is CuAg eutectic.
 4. The semiconductor package of claim 1,wherein the high thermal conductivity, low thermal expansion particlesare selected from the group consisting of diamond, cubic boron nitride,silicon carbide, and any combinations thereof.
 5. The semiconductorpackage of claim 1, wherein the base plate is entirely comprised of thecomposite.
 6. The semiconductor package of claim 1, wherein the baseplate comprises a core made of the composite and at least one outerlayer on the core.
 7. The semiconductor package of claim 6, furthercomprising one or more dice or transistors mounted on the at least oneouter layer.
 8. The semiconductor package of claim 7, wherein the coreis formed only in the region of the one or more dice or transistors. 9.The semiconductor package of claim 1, wherein the base plate comprises acore made of the composite.
 10. The semiconductor package of claim 8,further comprising an outer region surrounding the core.
 11. Thesemiconductor package of claim 9, further comprising at least one outerlayer on the core and the outer region.
 12. The semiconductor package ofclaim 9, wherein the outer region is a continuous phase of the silver orsilver alloy of the core.
 13. The semiconductor package of claim 9,wherein the silver comprises a silver alloy and the outer region is acontinuous phase of the silver alloy.
 14. The semiconductor package ofclaim 1, further comprising one or more dice or transistors mounted onthe base plate.
 15. The semiconductor package of claim 14, furthercomprising an insulated frame mounted on the base plate around the oneor more dice or transistors.
 16. The semiconductor package of claim 15,further comprising one or more conductive leads on the insulated frame.17. The semiconductor package of claim 1, further comprising aninsulated frame mounted on the base plate.
 18. The semiconductor packageof claim 17, further comprising one or more conductive leads on theinsulated frame.
 19. The semiconductor package of claim 4, wherein thehigh thermal conductivity, low thermal expansion particles are coatedwith one or more layers selected from the group consisting of Cr, W, Mo,Co, Cu, Ti, Si, SiC, TiN, TiC, Ta, Zr, and any combinations thereof. 20.A semiconductor package comprising a baseplate at least partially formedof a metal matrix composite consisting essentially of CuAg eutectic andmolybdenum coated diamond.
 21. The semiconductor package of claim 20,wherein the base plate is entirely comprised of the composite.
 22. Thesemiconductor package of claim 20, wherein the base plate comprises acore made of the composite and at least one outer layer on the core. 23.The semiconductor package of claim 22, wherein the core is formed onlyin a dice or transistor mounting region.
 24. The semiconductor packageof claim 20, wherein the base plate comprises a core made of thecomposite formed only in a dice or transistor mounting region and anouter region surrounding the core.
 25. The semiconductor package ofclaim 24, further comprising at least one outer layer on the core andthe outer region.
 26. The semiconductor package of claim 24, wherein theouter region is a continuous phase of the CuAg eutectic.
 27. Thesemiconductor package of claim 20, further comprising an insulated framemounted on the base plate around a dice or transistor mounting region.28. The semiconductor package of claim 27, further comprising one ormore conductive leads on the insulated frame.
 29. The semiconductorpackage of claim 1, wherein the metal matrix composite is formed bymixing the high thermal conductivity, low thermal expansion particleswith molten silver that has an oxygen content of less than 100 parts permillion.
 30. The semiconductor package of claim 1, wherein the metalmatrix composite is formed by mixing the high thermal conductivity, lowthermal expansion particles with molten silver that has an oxygencontent of less than 50 parts per million.
 31. The semiconductor packageof claim 20, wherein the metal matrix composite is formed by mixing themolybdenum coated diamond with CuAg eutectic that has an oxygen contentof less than 300 parts per million.
 32. The semiconductor package ofclaim 20, wherein the metal matrix composite is formed by mixing themolybdenum coated diamond with CuAg eutectic that has an oxygen contentof less than 50 parts per million.